Lead-free solder alloy and a manufacturing process of electric and electronic apparatuses using such a lead-free solder alloy

ABSTRACT

A lead-free solder alloy composition containing Sn, Ag and Bi, with respective concentrations set such that the lead-free solder alloy has a melting temperature lower than a predetermined heat-resistant temperature of a work to be soldered.

BACKGROUND OF THE INVENTION

[0001] The present invention generally relates to manufacturing ofelectric and electronic apparatuses and more particularly to a solderalloy of various forms used for soldering electric and electroniccomponents, as well as to a soldering process and further to a rig usedfor such a soldering process. In particular, the present inventionrelates to a lead-free solder alloy that contains no substantial amountof lead (Pb).

[0002] Solder alloys are characterized by low melting temperatures andprovide excellent electric as well as mechanical properties. Thus,solder alloys of various forms, including solder powders and solderpastes, are used for mounting electronic components on a printed circuitboard.

[0003] Meanwhile, conventional solder alloys contain Pb. As Pb is toxicagainst biological bodies, it has been necessary to take precautionarymeasure when conducting such a soldering process, while such aprecautionary measure increases the cost of the products produced as aresult of the soldering. Thus, there is a demand for a lead-free solderalloy that is suitable for use in various soldering processes includingautomated soldering process.

[0004] In the automated soldering process of electronic components,several types of solder alloys are used conventionally. A representativeexample is a solder alloy known as Sn63-Pb37, wherein the solder alloycontains 63 wt % of Sn and 37 wt % of Pb. This material causes aneutectic melting at a melting temperature of 183° C. Another typicalexample is a solder alloy known as Sn62-Pb36-Ag2, wherein the solderalloy contains 62 wt % of Sn, 36 wt % of Pb and 2 wt % of Ag. The solderalloy forms an eutectic system characterized by a melting temperature of179° C. As these solder alloys have low melting temperatures and provideexcellent mechanical properties in terms of tensile strength andelongation as well as excellent electrical properties such as lowresistance, they are used extensively for various automated solderingprocesses.

[0005] Meanwhile, there is a tendency of increasing public regulationsagainst the use of Pb in view of human health and in view ofenvironmental protection. Under such circumstances, various efforts havebeen made for developing a substitute solder alloy that is free from Pb.

[0006] As the material for use in assembling electric and electronicapparatuses, such a substitute solder alloy is required to have a lowmelting temperature such that the soldered electric or electroniccomponent experiences little degradation of performance caused by theheat at the time of soldering. Further, such a substitute solder alloyshould have an excellent mechanical strength comparable to that of aconventional solder alloy that contains Pb.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is a general object of the present invention toprovide a novel and useful solder alloy of various forms as well as asoldering process wherein the foregoing problems are eliminated.

[0008] Another and more specific object of the present invention is toprovide a solder alloy free from Pb and still having a sufficiently lowmelting temperature, high conductivity and high mechanical strength.

[0009] Another object of the present invention is to provide a lead-freesolder alloy composition comprising Sn, Bi and In, said solder alloycontaining Sn, Bi and In with respective concentrations set such thatsaid lead-free solder alloy composition has a melting temperature lowerthan a predetermined heat-resistant temperature of a work to besoldered.

[0010] Another object of the present invention is to provide a methodfor soldering a work, comprising the steps of:

[0011] reflowing a lead-free solder alloy containing therein Sn, Bi andIn with respective contents set such that said solder alloy has amelting temperature lower than a predetermined heat-resistanttemperature of said work, said step of reflowing including a step ofheating said solder alloy to a temperature higher than said meltingtemperature; and

[0012] cooling said work at a part where a soldering has been made tosolidify said lead-free solder alloy.

[0013] Another object of the present invention is to provide a lead-freesolder alloy composition containing: Bi with a concentration notexceeding 60.0 wt %; In with a concentration not exceeding 50.0 wt %;one or more elements selected from a group consisting of Ag, Zn, Ge, Ga,Sb and P, with a concentration equal to or larger than 1.0 wt % butlower than 5.0 wt %; and Sn as a balancing component of said lead-freesolder alloy.

[0014] Another object of the present invention is to provide a solderingprocess of a work, comprising the steps of:

[0015] reflowing a leadfree solder alloy containing therein: Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said solder alloy; and

[0016] cooling said work at a part where a soldering is made to solidifysaid lead-free solder alloy.

[0017] Another object of the present invention is to provide a lead-freesolder alloy composition containing Sn, Ag and Bi, with respectiveconcentrations set such that said lead-free solder alloy has a meltingtemperature lower than a predetermined heat-resistant temperature of awork to be soldered.

[0018] Another object of the present invention is to provide a method ofsoldering a work, comprising the step of:

[0019] reflowing a lead-free solder alloy containing therein Sn, Ag andBi with respective contents set such that said lead-free solder alloyhas a melting temperature lower than a predetermined heat-resistanttemperature of said work, said step of reflowing including a step ofheating said lead-free solder alloy to a temperature higher than saidmelting temperature; and

[0020] cooling said work at a part where a soldering is made to solidifysaid lead-free solder alloy.

[0021] Another object of the present invention is to provide a lead-freesolder powder comprising:

[0022] a plurality of lead-free solder particles each having a generallyspherical shape with a diameter of 20-60 μm;

[0023] each of said lead-free solder particles containing Sn, Bi and In,with respective concentrations set such that said lead-free solderparticle has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered.

[0024] Another object of the present invention is to provide a lead-freesolder powder comprising:

[0025] a plurality of lead-free solder particles each having a generallyspherical shape with a diameter of 20-60 μm;

[0026] each of said lead-free solder particles containing Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said lead-free solder particle.

[0027] Another object of the present invention is to provide a lead-freesolder powder comprising:

[0028] a plurality of lead-free solder particles each having a generallyspherical shape with a diameter of 20-60 μm;

[0029] each of said lead-free solder particles containing Sn, Ag and Bi,with respective concentrations set such that said lead-free solder alloyhas a melting temperature lower than a predetermined heat-resistanttemperature of a work to be soldered.

[0030] Another object of the present invention is to provide lead-freesolder paste, comprising:

[0031] a lead-free solder powder comprising a plurality of lead-freesolder particles each having a generally spherical shape with a diameterof 20-60 μm; each of said lead-free solder particles containing Sn, Biand In, with respective concentrations set such that said lead-freesolder particle has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered, said solder powderbeing contained with a proportion of 80.0-95.0 wt %; and

[0032] a mixture of an amine halide, a polyhydric alcohol and a polymer,with a proportion of 20.0-5.0 wt %.

[0033] Another object of the present invention is to provide a lead-freesolder paste, comprising:

[0034] a lead-free solder powder comprising a plurality of lead-freesolder particles each having a generally spherical shape with a diameterof 20-60 μm; each of said lead-free solder particles containing Bi witha concentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said solder alloy; said leadfree solder powder beingcontained with a proportion of 80.0-95.0 wt %; and

[0035] a mixture of an amine halide! a polyhydric alcohol and a polymer,with a proportion of 20.0-5.0 wt %.

[0036] Another object of the present invention is to provide a lead-freesolder paste, comprising:

[0037] a lead free solder powder comprising a plurality of lead-freesolder particles each having a generally spherical shape with a diameterof 20-60 μm; each of said lead-free solder particles containing Sn, Agand Bi, with respective concentrations set such that said lead-freesolder particle has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered; and

[0038] a mixture of an amine halide, a polyhydric alcohol and a polymer,with a proportion of 20.0-5.0 wt %.

[0039] Another object of the present invention is to provide a lead-freesolder paste, comprising:

[0040] a lead-free solder powder comprising a plurality of lead-freesolder particles each having a generally spherical shape with a diameterof 20-60 μm; each of said lead-free solder particles containing Sn, Biand In, with respective concentrations set such that said lead-freesolder powder has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered, said lead-freesolder powder being contained with a proportion of 80.0-95.0 wt %; and

[0041] a mixture of an organic acid, a polyhydric alcohol and a polymer,with a proportion of 20.0-5.0 wt %.

[0042] Another object of the present invention is to provide a lead-freesolder paste, comprising:

[0043] a lead-free solder powder comprising a plurality of lead-freesolder particles each having a generally spherical shape with a diameterof 20-60 μm; each of said lead-free solder particles containing Bi witha concentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said solder alloy; said leadfree solder powder beingcontained with a proportion of 80.0-95.0 wt %; and

[0044] a mixture of an organic acid, a polyhydric alcohol and a polymer,with a proportion of 20.0-5.0 wt %.

[0045] Another object of the present invention is to provide a lead-freesolder paste, comprising:

[0046] a lead-free solder powder comprising a plurality of lead-freesolder particles each having a generally spherical shape with a diameterof 20-60 μm; each of said lead-free solder particles containing Sn, Agand Bi, with respective concentrations set such that said lead-freesolder alloy has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered; and

[0047] a mixture of an organic acid, a polyhydric alcohol and a polymer,with a proportion of 20.0-5.0 wt %.

[0048] Another object of the present invention is to provide a printedcircuit board, comprising:

[0049] a substrate;

[0050] a conductor pattern provided on said substrate; and

[0051] a lead-free solder alloy covering said conductor pattern, saidlead-free solder alloy containing Sn, Bi and In, with respectiveconcentrations set such that lead-free said solder alloy has a meltingtemperature lower than a predetermined heat-resistant temperature of acomponent to be soldered upon said substrate.

[0052] Another object of the present invention is to provide printedcircuit board, comprising:

[0053] a substrate;

[0054] a conductor pattern provided on said substrate; and

[0055] a lead-free solder alloy covering said conductor pattern, saidlead-free solder alloy containing: Bi with a concentration not exceeding60.0 wt %; In with a concentration not exceeding 50.0 wt %; one or moreelements selected from a group consisting of Ag, Zn, Ge, Ga, Sb and P,with a concentration equal to or larger than 1.0 wt % but lower than 5.0wt %; and Sn as a remaining component of said lead-free solder alloy.

[0056] Another object of the present invention is to provide a printedcircuit board, comprising:

[0057] a substrate;

[0058] a conductor pattern provided on said substrate; and

[0059] a lead-free solder alloy covering said conductor pattern, saidlead-free solder alloy containing: Sn, Ag and Bi, with respectiveconcentrations set such that said lead-free solder alloy has a meltingtemperature lower than a predetermined heat-resistant temperature of acomponent to be soldered upon said substrate.

[0060] Another object of the present invention is to provide anelectronic component, comprising:

[0061] an electronic component body;

[0062] an electrode projecting from said electronic component body; and

[0063] a lead-free solder alloy covering said electrode, said lead-freesolder alloy containing Sn, Bi and In, with respective concentrationsset such that said lead-free solder alloy has a melting temperaturelower than a predetermined heat-resistant temperature of said electroniccomponent.

[0064] Another object of the present invention is to provide anelectronic component, comprising:

[0065] an electronic component body;

[0066] an electrode projecting from said electronic component body; and

[0067] a lead-free solder alloy covering said electrode, said lead-freesolder alloy containing: Bi with a concentration not exceeding 60.0 wt%; In with a concentration not exceeding 50.0 wt %; one or more elementsselected from a group consisting of Ag, Zn, Ge, Ga, Sb and Pi with aconcentration equal to or larger than 1.0 wt % but lower than 5.0 wt %;and Sn as a remaining component of said lead-free solder alloy.

[0068] Another object of the present invention is to provide anelectronic component, comprising:

[0069] an electronic component body;

[0070] an electrode projecting from said electronic component body; and

[0071] a lead-free solder alloy covering said conductor pattern, saidlead-free solder alloy containing: Sn, Ag and Bi, with respectiveconcentrations set such that said lead-free solder alloy has a meltingtemperature lower than a predetermined-heat-resistant temperature of acomponent to be soldered upon said substrate.

[0072] Another object of the present invention is to provide anelectronic apparatus, comprising:

[0073] a substrate;

[0074] a conductor pattern provided on said substrate;

[0075] an electronic component mounted upon said substrate in electricalconnection with said conductor pattern, said electronic component havingan electrode projecting therefrom; and

[0076] a lead-free solder alloy connecting said electrode to saidconductor pattern, said lead-free solder alloy containing Sn, Bi and In,with respective concentrations set such that said lead-free solder alloyhas a melting temperature lower than a predetermined heat-resistanttemperature of said electronic component.

[0077] Another object of the present invention is to provide anelectronic apparatus, comprising:

[0078] a substrate;

[0079] a conductor pattern provided on said substrate;

[0080] an electronic component mounted upon said substrate in electricalconnection with said conductor pattern, said electronic component havingan electrode projecting therefrom; and

[0081] a lead-free solder alloy connecting said electrode to saidconductor pattern, said lead-free solder alloy containing: Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said lead-free solder alloy.

[0082] Another object of the present invention is to providean-electronic apparatus, comprising:

[0083] a substrate;

[0084] a conductor pattern provided on said substrate;

[0085] an electronic component mounted upon said substrate in electricalconnection with said conductor pattern, said electronic component havingan electrode projecting therefrom; and

[0086] a lead-free solder alloy connecting said electrode to saidconductor pattern, said lead-free solder alloy containing Sn, Ag and Bi,with respective concentrations set such that said lead-free solder alloyhas a melting temperature lower than a predetermined heat-resistanttemperature of said electronic component.

[0087] Another object of the present invention is to provide a solderingrig for soldering a work, comprising:

[0088] soldering unit for soldering a work by causing a reflow of alead-free solder; and

[0089] a cooling unit for cooling said work at a part where a solderinghas been made, to solidify said lead-free solder.

[0090] According to the present invention as set forth above, one canobtain a solder alloy free from Pb while maintaining excellentmechanical strength in the solidified solder alloy. Thereby, the problemof hazard to biological bodies as well as the problem of environmentalpollution are successfully eliminated. Further, by optimizing thecomposition of the solder alloy, it is possible to reduce the meltingtemperature of the solder alloy lower than a melting temperature of aconventional solder alloy that contains Pb, while maintaining sufficientmechanical strength. Thereby, the damage applied to the work orelectronic component as a result of soldering is reduced. Associatedwith the reduced temperature of soldering, the preparation of the workfor soldering is substantially simplified, and the cost of the work isalso reduced by using less expensive materials. By cooling the solderalloy rapidly, it is possible to maximize the elongation of thesolidified solder alloy.

[0091] Other objects and further features of the present invention willbecome apparent from the following detailed description when read inconjunction with the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0092]FIG. 1 is a diagram summarizing the effect of In added to a solderalloy of a Sn—Bi eutectic system in the form of a table;

[0093]FIG. 2 is a diagram summarizing the effect of Bi added to a solderalloy of a Sn—Bi—In ternary system in the form of a table;

[0094]FIG. 3 is a diagram summarizing the effect of various elementsadded to a solder alloy of a Sn—Bi—In ternary system in the form of atable;

[0095]FIG. 4 is a diagram summarizing the effect of Bi added to a solderalloy of a Sn—Ag eutectic system in the form of a table;

[0096] FIGS. 5A-5I are diagrams showing a particle of a lead-free solderpowder according to an embodiment of the present invention;

[0097]FIG. 6 is a diagram showing the composition of a lead-free solderpaste according to another embodiment of the present invention in theform of a table;

[0098]FIG. 7 is a diagram showing the composition of a lead-free solderpaste according to other embodiment of the present invention in the formof a table;

[0099]FIG. 8 is a diagram showing the construction of a printed circuitboard according to still other embodiment of the present invention;

[0100]FIG. 9 is a diagram showing the construction of a printed circuitboard according to still other embodiment of the present invention;

[0101]FIG. 10 is a diagram showing the construction of a semiconductordevice mounted upon a printed circuit board according to still otherembodiment of the present invention;

[0102]FIG. 11 is a diagram showing the mechanical property of thelead-free alloy of various embodiments of the present invention in theform of a table;

[0103]FIG. 12 is a diagram showing the detailed experimental resultconducted for a sample included in FIG. 11, in the form of a table;

[0104]FIG. 13 is a diagram showing the detailed experimental resultconducted for another sample included in FIG. 11, in the form of atable;

[0105]FIG. 14 is a diagram showing the detailed experimental resultconducted for other sample included FIG. 11, in the form of a table;

[0106]FIG. 15 is a diagram showing the detailed experimental resultconducted for other sample included in FIG. 11, in the form of a table;

[0107]FIG. 16 is a diagram showing the detailed experimental resultconducted for other sample included in FIG. 11, in the form of a table;

[0108]FIG. 17 is a diagram showing the relationship between theelongation and the load for the sample of FIG. 12;

[0109]FIG. 18 is a diagram showing the relationship between theelongation and the load for the sample of FIG. 13;

[0110]FIG. 19 is a diagram showing the relationship between theelongation and the load for the sample of FIG. 14;

[0111]FIG. 20 is a diagram showing the relationship between theelongation and the load for the sample of FIG. 15;

[0112]FIG. 21 is a diagram showing the relationship between theelongation and the load for the sample of FIG. 16;

[0113]FIG. 22 is a diagram showing the state of fracture of the sampleof FIG. 12;

[0114]FIG. 23 is a diagram showing the state of fracture of the sampleof FIG. 13;

[0115]FIG. 24 is a diagram showing the state of fracture of the sampleof FIG. 14;

[0116]FIG. 25 is a diagram showing the state of fracture of the sampleof FIG. 15;

[0117]FIG. 26 is a diagram showing the state of fracture of the sampleof FIG. 16;

[0118]FIG. 27 is a diagram showing the soldering process according tostill other embodiment of the present invention in the form of aflowchart;

[0119]FIG. 28 is a diagram showing a soldering rig according to a stillother embodiment of the present invention;

[0120]FIG. 29 is a diagram showing the construction of a cooling unit ofthe soldering rig of FIG. 28;

[0121]FIG. 30 is a diagram showing the construction of another coolingunit of the soldering rig of FIG. 28; and

[0122]FIG. 31 is a diagram showing the construction of still othercooling unit of the soldering rig of FIG. 28.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0123] Hereinafter, the present invention will be described withreference to the preferred embodiments.

[0124] In the present invention, the inventor has conducted a series ofexperiments to prepare various solder alloys free from Pb and to examinethe properties thereof. As a result, it has been discovered that alead-free solder alloy containing Sn, Bi and In as well as a lead-freesolder alloy containing Sn, Ag and Bi, show mechanical as well aselectrical properties comparable or even superior to those of theconventional solder alloy that contains Pb.

[0125] Thus, the inventor of the present invention has conducteddetailed experiments on the lead-free solder alloy containing Sn, Bi andIn as well as on the lead-free solder alloy containing Sn, Ag and Bi insearch of the optimum composition of the solder alloy. Further,experiments have been conducted also on the alloys in which other metalor non-metal elements are added.

[0126] First, a description will be made on the experiments about thesolder alloy of the Sn—Bi—In ternary system with reference to FIGS. 1and 2, wherein FIGS. 1 and 2 are tables that summarize the result of theexperiments on the tensile strength, percentage of elongation,time-to-failure, fracture surface morphology and the melting temperaturefor various compositions of the solder alloy. Further, FIG. 3 shows atable summarizing the result of the similar experiments conducted forsolder alloys in which other metal as well as non-metal elements areadded.

[0127] Before going to the evaluation of the experimental results,explanation will be made on the testing process and testing apparatusesused in the experiments.

[0128] In the experiments, standard test pieces prescribed in the JIS(Japanese Industrial Standard) were produced from solder alloys ofvarious compositions, and the test pieces thus produced were subjectedto a tensile test for mechanical properties such as tensile strength,percentage of elongation, time-to-failure and fracture surfaceobservation. Further, the melting temperature of the solder alloy wasmeasured by using a thermocouple.

[0129] More specifically, the test pieces were produced according to theJIS type 7 prescription for tensile tests. The test piece thus producedhad a cross sectional area of 40 mm² and a gauge length of 30 mm. Thetest pieces were prepared by melting an alloy of Sn, Bi and In or analloy containing further impurity elements in a furnace held at 400° C.,wherein the molten solder alloy thus prepared was poured into a moldcarrying therein a cavity with a shape corresponding to the JIS type 7test piece prescription.

[0130] In the tensile test, a test rig of Instron Model 4206 was used.The test piece was set on the rig, and the test was conducted by pullingthe test piece with a fixed rate of 0.5 mm/min while recording thetensile load and the elongation of the test piece. Upon occurrence ofthe failure, the tensile strength and the percentage of elongation werecalculated based upon the record. Further, a discrimination was madewhether the fracture was a ductile one or brittle one based upon theobservation of the fracture surface morphology.

[0131] The measurement of the melting temperature or liquidustemperature of the solder alloy, on the other hand, was made by causinga melting of the solder alloy, followed by a natural cooling. During theprocess of natural cooling, the temperature profile was measured bymeans of a thermocouple inserted into the molten solder alloy.

[0132] As already noted, FIGS. 1 through 3 summarize the result of thetest in terms of the tensile strength, elongation, time-to-failure,fracture surface and the melting temperature. When the propertiesobserved were satisfactory for a solder alloy, an open circle mark wasgiven as an indication of positive evaluation. When the properties wereunsatisfactory, on the other hand, a cross mark was given as anindication of negative evaluation. In the present test, the evaluationwas made based upon a standard that: (1) the solder alloy should have atensile strength of at least 2 kg/mm²; (2) the solder alloy should havean elongation of at least 30%; and (3) the solder alloy should have amelting temperature equal to or lower than 155° C.

[0133] Referring to FIG. 1, it will be noted chat the content of In ischanged variously in the ternary alloy of the Sn—Bi—In system whilemaintaining the content of Bi generally constant. As will be noted inFIG. 1, the solder alloy provides a satisfactory tensile strength aslong as the In content in the alloy is less than 50 wt %. When the Incontent exceeds 50 wt % as in the case of the comparative examples 3 and4, on the other hand, the solder alloy fails to provide a satisfactorytensile strength. About the elongation, it should be noted that thesolder alloy containing In with a content less than 0.5 wt % as in thecase of the comparative examples 1 and 2 does not provide a satisfactoryresult, while the solder alloy containing In with a content of 0.5 wt %or more provides a satisfactory result.

[0134] About the melting temperature, all of the test samples in FIGS.1-3 satisfy the requirement that the melting temperature should be lowerthan 155° C. It should be noted that electronic components are generallydesigned to have a heat-resistant temperature of 183° C. in view of useof conventional solder alloy that contains Pb. By using the solder alloyof the present invention, on the other hand, it is possible to conductthe soldering process at a temperature lower than the temperature usedconventionally, and the problem of thermal damage to the electroniccomponents is minimized. Further, it is possible to reduce the cost ofthe electronic apparatus by simplifying the preparation process ofsoldering as well as by using inexpensive materials for the electroniccomponents.

[0135] It should be noted that FIG. 1 further shows a tendency that themelting temperature of the solder alloy decreases with increasing Incontent. In other words, FIG. 1 indicates that one can control themelting temperature of the solder alloy by controlling the In content.Thereby, any necessary change of the soldering temperature dependingupon the electronic component such as a semiconductor device, is easilyattended to.

[0136] With regard to the time-to-failure, it will be noted that nosatisfactory result is obtained when the In content is less than 0.5 wt% as in the case of the comparative examples 1 and 2 or when the Incontent exceeds 50 wt % as in the case of the comparative example 3. Itis noted that there is an exception in the case of the comparativeexample 4 in which the time-to-failure falls in the satisfactory rangeof 40-60 minutes even when the In content exceeds 50 wt %. It isbelieved that this exception is caused because of the Bi-freecomposition of the solder alloy that has resulted in an increase of thetime-to-failure.

[0137] With regard to the fracture surface of the tested samples, it isnoted that a brittle fracture occurs when the In content of the solderalloy is less than 5.0 wt %. When the In content is equal to or largerthan 5.0 wt %, a ductile fracture occurs. While the type of fracture ofthe solder alloy may not affect the property thereof as a solder, it ismore preferable that the solder alloy shows ductile fracture thanbrittle fracture in view point of the mechanical strength.

[0138] Summarizing the result of FIG. 1, it is concluded that one canobtain a solder alloy of desirable property by incorporating In into asolder alloy of the Sn—Bi eutectic system with a proportion of 0.5 wt %or more but less than 50.0 wt %. In FIG. 1, the samples 1-1-1-5 providesuch desirable properties.

[0139] In the sample 1-5, it should be noted that, while the tensilestrength is slightly larger than the acceptable lower limit, the solderalloy provides a much larger elongation over other samples as well ascomparative examples cited in the table of FIG. 1. Thus, by setting thecomposition of the solder alloy such that the solder alloy contains Snwith a proportion of about 34.0 wt %, Bi with a proportion of about 46.0wt % and In with a proportion of about 20.0 wt %, it is possible toobtain a solder alloy having an excellent elongation. Such a solderalloy composition is particularly suitable for soldering components upona flexible substrate where the solder alloy experiences largedeformation.

[0140] In the description above, the representation of the compositionsuch as “about 34.0 wt %,” “about 20.0 wt %,” and the like, is used, inview of possible error in the composition of the solder alloy that canreach as much as ±1 wt % for Sn and Bi and ±0.1 wt % for In.

[0141] Next, the result of FIG. 2 will be explained. As already noted,FIG. 2 shows the properties of various solder alloys all included in theternary eutectic system of Sn—Bi—In but with various Bi contents and agenerally common In content.

[0142] Referring to FIG. 2, it will be noted that a satisfactory tensilestrength is obtained when the Bi content has exceeds 5.0 wt %. About theelongation, no satisfactory result is obtained when the Bi content isequal to or larger than 60.0 wt % as in the case of the comparativeexamples 12-15, while the solder alloy containing Bi with a proportionless than 60.0 wt % provides a satisfactory elongation. About themelting temperature, all of the solder alloy compositions, except forthe example in which the Bi content is 100%, satisfy the requirement. Inother words, it is demonstrated that the melting temperature or liquidustemperature is reduced in the solder alloy that contains Sn, Bi and In.

[0143] The result of FIG. 2 further indicates the tendency that themelting temperature increases with increasing Bi content. Thus, byadjusting the Bi content, it is possible to control the meltingtemperature of the solder alloy.

[0144] About the time-to-failure, it will be noted that the solderalloys containing Bi with less than 5.0 wt %, as in the comparativeexamples 10 and 11, as well as the solder alloys containing Bi with 60.0wt % or more, as in the case of the comparative experiments 12-15,provide a reduced time-to-failure and hence an unsatisfactorydurability. Further, the observation of the fracture surface indicatesthat the solder alloy shows a ductile fracture when the Bi content isless than 55.0 wt %, while the fracture becomes brittle when the Bicontent in the solder alloy is equal to or larger than 55.0 wt %.

[0145] Summarizing the result of FIG. 2, it is concluded that a solderalloy suitable for soldering electric and electronic components isobtained by adding Bi to a solder alloy of the Sn—In eutectic system,with a proportion that exceeds 5.0 wt % but smaller than 60.0 wt % as incase of the examples 2-1 and 2-2 of FIG. 2.

[0146] Further, the results of FIGS. 1 and 2 collectively indicate thata solder alloy suitable for soldering electric and electronic componentsis obtained by setting the Bi content to be less than 60.0 wt %, the Incontent less than 50.0 wt %, and by balancing the rest of the solderalloy by Sn. Particularly, one obtains a solder alloy of optimumcomposition by setting the Sn content to about 40.0 wt %, the Bi contentto about 55.0 wt %, and the In content to about 5.0 wt %. As alreadynoted, the phrase “about” is used in view of the possible error in thecomposition when forming the alloy. The error can be as large as ±1 wt %for Sn and Bi and ±0.1 wt % for In.

[0147] Next, a description will be made on the experiments conducted bythe inventor with reference to FIG. 3, wherein FIG. 3 shows the resultof the experiments conducted upon a solder alloy based upon the Sn—Bi—Ineutectic system, except that other metal elements, particularly Ag andZn, are added to the foregoing ternary system.

[0148] It will be noted that the solder alloy does not satisfy therequirement about elongation when Ag and Zn are added to the solderalloy of the Sn—Bi—In ternary eutectic system with a proportion of 5.0wt % or more for each of Ag and Zn. On the other hand, when theproportion of one of Ag and Zn is set to 1.0 wt %, the requirement forelongation is satisfied.

[0149] Further, it will be noted in FIG. 3 that all of the samplessatisfy the requirement about melting temperature. The result of FIG. 3indicates that one can reduce the melting temperature and hence theliquidus temperature by incorporating Ag and Zn to the ternary solderally of the Sn—Bi—In eutectic system.

[0150] The result of FIG. 3 clearly indicates that the solder alloycontaining Ag and Zn with a proportion of 1.0 wt % or more but below 5.0wt %, such as the examples 3-1 and 3-2, satisfies the requirementimposed upon a solder alloy, with every respect of the requirement.Further, it should be noted that the content of Bi and In in FIG. 3falls in the optimum range derived from the result of FIGS. 1 and 2. Inother words, the content of Bi does not exceed 60.0 wt % and the contentof In does not exceed 50.0 wt %.

[0151] Summarizing the result of FIG. 3, a solder alloy suitable forsoldering is obtained from a ternary alloy of the Bi—In—Sn eutecticsystem by setting the proportion of Bi and In such that the Bi contentdoes not exceed 60.0 wt %, the In content does not exceed 50.0 wt % andby incorporating Ag or Zn with a proportion equal to or larger than 1.0wt % but smaller than 5.0 wt %. The rest of the alloy composition isbalanced by Sn. In the embodiment of FIG. 3, it should be noted thatother metal elements such, as Ge or Ga may be used in place of Ag andZn. Further, non-metal elements such as P may also be used for thispurpose.

[0152] In the ternary alloy composition of the Sn—Bi—In eutectic systemshown in FIG. 3, it is also possible to incorporate Sb as an additionalmetal element. By adding Sb, the problem of elemental diffusion to aSn—Pb plating is successfully eliminated. When Pb and Bi are contactedwith each other, there tends to occur a problem of diffusion, which inturn results in a bulging or coming-off of the solder metal. Thereby,the reliability of the soldering deteriorates significantly. In thelead-free solder alloy of the present invention, such a problem ofdegradation of the solder alloy is successfully eliminated byincorporating Sn as noted above. Thereby, it is preferable to controlthe Sn content in the solder alloy to fall in the range of 1.0-5.0 wt %.The Sn content is optimized in this range in view of the tensilestrength and the elongation of the solder alloy.

[0153] Next, a solder alloy of the Sn—Ag—Bi system will be describedwith reference to FIG. 4 that shows the result of the test conductedupon the tensile strength, elongation, time-to-failure, fracture surfacemorphology and the melting temperature for the solder alloy of variouscompositions. As the tests conducted upon the solder alloys of FIG. 4are identical with the tests described already, further descriptionthereof will be omitted. Similarly to FIGS. 1-3, FIG. 4 cites theevaluation about the tensile strength, elongation, time-to-fracture andthe melting temperature. When the evaluation is positive, a designationis made by an open circle mark. When the evaluation is negative, on theother hand, a designation is made by a cross mark.

[0154] In the test of FIGS. 4, a standard is imposed such that asatisfactory solder alloy should have a tensile strength of 7 kg/mm² ormore, an elongation of 7.0% or more and a melting temperature of 220° C.or less. It will be noted that this standard is different from thestandard applied to the solder alloy containing Sn, Bi and In. Thereason of using a such different standard is to meet the demand for asolder alloy having a particularly large tensile strength. Such a demandon the other hand does not require a high elongation as in the case ofthe foregoing solder alloy of the Sn—Bi—In system.

[0155] Referring to FIG. 4, the table shows the properties of theternary solder alloy of the Sn—Ag—Bi eutectic system for variouscontents of Bi while maintaining the Ag content substantially constant.

[0156] From the result of FIG. 4, it will be noted that the solder alloyof the examples 4-1-4-6 satisfies the foregoing standard. On the otherhand, the comparative example 30 that contains Pb has a lower tensilestrength and does not satisfy the foregoing standard, contrary to theternary alloy of the Sn—Ag—Bi system. A similar result was obtained alsofor the example 31 for the binary alloy of the Sn—Bi eutectic system andfor the example 32 for the binary alloy of the Sn—Ag eutectic system.

[0157] About the observed elongation, all of the examples of FIG. 4satisfy the required standard. It is known that the elongation and thetensile strength tend to contradict with each other. Thus, there is atendency that an alloy having a large tensile strength shows a smallelongation. Under such circumstances, the solder alloy of the presentembodiment provides a particularly high tensile strength whilesacrificing the elongation. For example, the samples 4-1-4-6 shows atensile strength higher than that of the comparative examples 30-32 andan elongation smaller than that of the comparative examples 30-32.

[0158] With regard to the melting temperature, all of the examples shownin the table of FIG. 4 satisfy the required standard. Particularly, theexamples 4-1-4-6 show a melting temperature falling in the range of 139°C.-220° C. It has been practiced, in the conventional lead-containingsolder alloys, to adjust the composition of the alloy such that themelting temperature is held low in view of the endurable temperature of183° C. of the electronic components to be soldered. On the other hand,there also are demands for a solder alloy composition having a highermelting temperature such as 220° C. or more. In order to meet such ademand, there exist a group of solder alloys in which the meltingtemperature is adjusted higher than 220° C. According to the presentembodiment as set forth in the table of FIG. 4, one can provide alead-free solder alloy that is suitable for the purpose whilesimultaneously maintaining a sufficient tensile strength.

[0159] With regard to the time-to-failure, there is a tendency that thetime-to-failure increases with increasing elongation. Thus, the examples4-1-4-6 provide a relatively small time-to-failure value as comparedwith the comparative examples 30-32. Even then, the solder alloys of theexamples 4-1-4-6 provides a satisfactory time-to-failure of 230-670seconds while maintaining a large tensile strength.

[0160] With regard to the fracture surface, the solder alloy of thepresent embodiment shows a feature of brittle fracture. As the solderalloy of the present embodiment is intended to provide a high tensilestrength, the evidence that the solder alloy shows a brittle fracturedoes not cause any serious consequence.

[0161] Summarizing the result of FIG. 4, it will be noted that theternary solder alloy of the Sn—Ag—Bi system provides a superior tensilestrength over the eutectic solder alloys of other compositions describedpreviously such as the examples 30-32, as clearly demonstrated in theexamples 4-1-4-6. Further, it is possible to adjust the meltingtemperature as desired within the temperature range conventionally usedfor soldering. Thus, it is possible to carry out the soldering atvarious temperatures optimized for the components to be soldered whilesimultaneously maintaining a high tensile strength, by selecting thecomposition of the lead-free solder alloy according to the purpose.

[0162] In the examples 4-1-4-6 of FIG. 4, it should be noted that theratio of the Sn wt % to the Ag wt % is held constant and only the Bicontent is changed. In other words, the solder alloy composition of theexamples 4-1 4-6 is represented as 96.5×(100−X)/100 for Sn,3.5×(100−X)/100 for Ag, and X for Bi, all represented in terms of wt %.

[0163] From FIG. 4, it is concluded that the following compositions aresuitable for the solder alloy having a large tensile strength: a solderalloy containing Ag with an amount not exceeding 4.0 wt %, Bi with anamount equal to or larger than 1.0 wt %, and Sn with an amount notexceeding 95.0 wt %; a solder alloy containing Ag with an amount between1.0 wt % and 4.0 wt %, Bi with an amount between 1.0 wt % and 40.0 wt %,and Sn with an amount between 55.0 wt % and 95.0 wt %, a solder alloycontaining Ag with an amount of approximately 3.3 wt %, Bi with. anamount of approximately 5.0 wt %, and Sn with an amount of approximately91.7 wt %; a solder alloy containing Ag with an amount of approximately3.1 wt %, Bi with an amount of approximately 10.0 wt %, and Sn with anamount of approximately 86.9 wt %; a solder alloy containing Ag with anamount of 3.0 wt %, Bi with an amount of 15.0 wt %, and Sn with anamount of 82.0 wt %; a solder alloy containing Ag with an amount of 2.8wt %, Bi with an amount of 20.0 wt %, and Sn with an amount of 77.2 wt%; a solder alloy containing Ag with an amount of 2.4 wt %, Bi with anamount of 30.0 wt %, and Sn with an amount of 67.6 wt %; a solder alloycontaining Ag with an amount of 2.1 wt %, Bi with an amount of 40.0 wt%, and Sn with an amount of 57.9 wt %, and the like.

[0164] FIGS. 5A-5I are diagrams showing the examples of solder particlesforming a solder powder.

[0165] Referring to FIG. 5A, the solder alloy of the examples 1-1 -1-5shown in FIG. 1 or the solder alloy of the examples 2-1 or 2-2 of FIG.2, is used to form a generally spherical solder particle having adiameter of 20-60 μm. FIG. 5B, on the other hand, shows a solderparticle formed of the solder alloy of the example 3-1 or 3-2 of FIG. 3,wherein the solder particle has a generally spherical form and adiameter of 20-60 μm. Further, FIG. 5C shows a solder particle formed ofthe solder alloy of the examples 4-1-4-6 of FIG. 4, wherein the solderparticle has a generally spherical form and a diameter of 20-60 μm.

[0166]FIG. 5D, on the other hand, shows a composite solder particle, inwhich a core particle, formed of the solder alloy of any of the examples1-1-1-5 of FIG. 1 or 2-1-2-2 of FIG. 2, is covered by Sn or an alloy ofSn containing Ge with a proportion of 0.1-5.0 wt %, wherein thecomposite solder particle as a whole has a generally spherical form anda diameter of 20-60 μm.

[0167]FIG. 5E, on the other hand, shows another composite solderparticle; in which a core particle, formed of the solder alloy of any ofthe examples 3-1 and 3-2 of FIG. 3, is covered by a similar alloy thatcontains Sn or Ge further with a proportion of 0.1-5.0 wt %, wherein thecomposite solder particle as a whole has a generally spherical form anda diameter of 20-60 μm.

[0168] Further, FIG. 5F shows another composite solder particle, inwhich a core particle of the solder alloy of any of the examples 4-1-4-6of FIG. 4, is covered by a similar alloy that contains Sn or Ge furtherwith a proportion of 0.1-5.0 wt %, wherein the composite solder particleas a whole has a generally spherical form and a diameter of 20-60 μm.

[0169]FIG. 5G shows another composite solder particle, in which a coreparticle of the solder alloy of any of the examples 1-1-1-5 of FIG. 1 orthe examples 2-1 and 2-2 of FIG. 2 is covered by a similar alloycontaining Sn and Bi with respective proportions exceeding 20.0 wt % andless than 60.0 wt %, wherein the composite solder particle as a wholehas a generally spherical form and a diameter of 20-60 μm.

[0170]FIG. 5H shows a still other composite solder particle, in which acore particle of the solder alloy of any of the examples 3-1 and 3-2 ofFIG. 3 is covered by a similar alloy that contains Sn and Bi withrespective proportions exceeding 20.0 wt % and less than 60.0 wt %,wherein the composite solder particle as a whole has a generallyspherical form and a diameter of 20-60 μm.

[0171]FIG. 5I shows a still other composite solder particle, in which acore particle of the solder alloy of any of the examples 4-1-4-6 of FIG.4 is covered by a similar alloy containing Sn and Bi with respectiveproportions exceeding 20.0 wt % and less than 60.0 wt %, wherein thecomposite solder particle as a whole has a generally spherical form anda diameter of 20-60 μm.

[0172] In any of the embodiments in FIGS. 5A-5I, it is possible to forma solder paste from the solder powder formed of the solder particles.Further, in the embodiments of FIGS. 5D-5I, it is possible to eliminatethe problem of oxidation of the solder alloy by covering the solderalloy by an alloy containing Sn or Ge with a proportion of 0.1-0.5 wt %or by an alloy containing Sn and Bi with the proportion of Sn exceeding20.0 wt % and the proportion of Bi not exceeding 60.0 wt %.

[0173] Hereinafter, the solder paste that uses the solder powder of theprevious embodiments will be described.

[0174] The inventor of the present invention has prepared various solderpaste compositions, the first series of compositions being a mixture ofa solder powder, an amine halide, a polyhydric alcohol and a polymercompound, wherein first series composition contains the solder powderwith a proportion of 80.0-95.0 wt %. Thus, the solder paste compositionof the first series contains, as the remaining component, the aminehalide, the polyhydric alcohol and the polymer compound with aproportion of 20.0-5.0 wt %. The second series composition is a mixtureof a solder powder, an organic acid, polyhydric alcohol and a polymercompound, wherein the second series composition contains the solderpowder with a proportion of 80.0 wt %-95.0 wt %. Thus, the solder pastecomposition of the second series contains, as the remaining component,the organic acid, the polyhydric alcohol and the polymer compound with aproportion of 20.0-5.0 wt %.

[0175] As the amine halide for use in the solder paste, one may selectone or more from the group of: acrylic amine hydrochloride, anilinehydrochloride, diethylamine hydrochloride, cyclohexylaminehydrochloride, monomethylamine hydrochloride, dimethylaminehydrochloride, trimethylamine hydrochloride, phenylhydrazinehydrochloride, n-butylamine hydrochloride, O-methylhydrazinehydrochloride, ethylamine oxalate, cyclohexyl oxalate,2-aminoethylbromide oxalate, and tri-n-butylamine oxalate.

[0176] As the organic acid for use in the solder paste, one may selectone or more from the group of: oxalic acid, malonic acid, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, maleic acid, tartaric acid, benzoic acid, acetic acid,hydroxyacetic acid, propionic acid, butyric acid, n-veleric acid,n-caproic acid, enanthic acid, n-capric acid, lauric acid, myristicacid, palmitic acid, stearic acid, and the like.

[0177]FIG. 6 shows an example of the solder paste composition that usesa solder alloy described in one of the examples shown in FIGS. 1 through3 as a solder powder, while FIG. 7 shows an example of the solder pastecomposition that uses a solder alloy described in one of the examplesshown in FIG. 4.

[0178] It should be noted that the amine halides or organic acidsdescribed above act as an activating agent. Further, one may use abieticacid, dehydroabietic acid, α-terpineol, and the like, for the base ofthe paste. The solder paste may further contain a polymer compound suchas cured castor oil as a thixotropic agent. Further, a polyhydricalcohol such as 2-methyl 2,4-pentadiol may be added as a solvent.

[0179] The solder paste composition described above is naturally freefrom Pb and can be used for a hazard-free reflowing process that doesnot require precautionary measure against toxicity of Pb. Thereby, theefficiency of production of the electronic apparatuses is improved.

[0180]FIG. 8 shows an example of application of the lead-free solderalloy upon a conductor pattern on a printed circuit board.

[0181] Referring to FIG. 8, a printed circuit board 1 includes a basemember 2 of a glass-epoxy, wherein the base member 2 carries thereon anelectrode 3 of Cu for external connection. On the electrode 3, aleadfree solder alloy selected from any of the examples described inFIGS. 1-3 is applied, such that a film 4 of the solder alloy covers theelectrode 3. It was confirmed that such a construction provides anexcellent junction or adherence between the solder alloy and theelectrode 3 of Cu, and the film of the solder alloy 4 covers theelectrode 3 uniformly.

[0182]FIG. 9 shows an example in which the solder alloy is applied tocoat a lead 7 of an electronic component 5 that may be a semiconductordevice having a resin package body 6.

[0183] Referring to FIG. 9, the lead 7 may be formed of any of Cu,42-alloy (containing Ni 42 wt %, Co 0.5 wt %, Mn 0.8 wt % and balancingFe) and Covar, and the lead-free solder alloy of various compositionsselected from the examples in FIGS. 1-3 covers the lead 7. It wasconfirmed that such a construction provides an excellent junction oradherence between the lead 7 and the solder alloy, and a solder alloyfilm 8 is formed on the lead 7 with a uniform thickness.

[0184] As described above, the lead-free solder alloy successfully coatthe conductor patterns on the printed circuit board as well as theterminals of electronic components, and one can achieve a reliableelectrical as well as mechanical connection between the electroniccomponents and the printed circuit board.

[0185]FIG. 10 shows an example of using the lead-free solder alloy ofthe present invention for the solder bumps that form an externalconnection terminal of an electronic apparatus such as a semiconductordevice 9.

[0186] Referring to FIG 10, the semiconductor device 9 includes asubstrate 13 carrying thereon a semiconductor element not illustrated,wherein the semiconductor element is embedded in a resin package body 14provided on the substrate 13. Further, the substrate 13 carries aplurality of solder bumps 11 on a lower major surface thereof forexternal connection. Upon placing the semiconductor device 9 on aprinted circuit board 12, the solder bumps 11 engage with correspondingconductor patterns provided on the printed circuit board 12. Thus, thereoccurs a reflowing of the solder bumps 11 upon passage of the printedcircuit board 12 through a furnace, and a reliable electrical as well asmechanical connection is achieved thereby according to the flip-chipprocess, without using a lead-containing solder alloy.

[0187] Next, the soldering process as well as the soldering rigdeveloped for the lead-free solder alloy of the present invention willbe described.

[0188] In the foregoing experimental result summarized in FIGS. 1-4, itwill be noted that there are examples that show anomalously largeelongation as in the case of the examples 1-5 of FIG. 1, 3-2 of FIG. 3and 4-1 of FIG. 4. Further, it was rather frequently observed that thelead-free solder alloys containing Sn and Bi show a rather remarkableincrease of elongation.

[0189] The inventor of the present invention at first attributed thiseffect to the effect of the impurities contained in the solder alloy.Thus, a chemical analysis was conducted upon the solder alloys thatshowed anomalous elongation by way of the XRF (X-ray fluorescent)analysis and by way of the induction plasma spectroscopy. The result ofthe chemical analysis, however, clearly showed that the solder alloy isessentially formed of Sn and Bi and that there is no substantialcontamination of the solder alloy by a third element.

[0190] Based upon the result of the chemical analysis above, theinventor of the present invention has set a working hypothesis that theanomalous elongation occurs as a result of the process of preparing thetest specimen, particularly the cooling rate when molding the test pieceused for the test piece of the specimen.

[0191] Thus, the inventor has conducted a series of experiments to moldthe test pieces with various cooling rates, and test the pieces thusformed were subjected to tests for various mechanical properties such astensile strength, elongation, time-to-fracture, fracture surfaceobservation as well as tests for various metallurgical properties suchas the surface state and metallurgical texture. In the experiments, abinary eutectic alloy having a composition of 42.0 wt % for Sn and 58.0wt % for Bi was used throughout.

[0192] When molding the test pieces, three different cooling processes,i.e., natural cooling process, water cooling process and gradual coolingprocess, were employed. In the natural cooling process, a molten alloywas left in the room temperature environment together with a mold.Thereby, test pieces 5-1-5-3 were obtained according to such a naturalcooling process. In the water cooling process, the mold was cooledcompulsorily by water after molding the test piece from the moltensolder alloy. Thereby, a test piece 5-4 was obtained. In the gradualcooling process, the molten alloy in the mold was gradually cooled byholding the mold in a thermally insulated environment. Thereby, a testpiece 5-5 is obtained.

[0193] By employing various cooling processes, the cooling rate of thesolder alloy at the time of molding the test piece is changed variously.Particularly, the molding according to the natural cooling process isconducted by setting the mold at various initial temperatures such as200° C. in the case of the example 5-1, 100° C. in the case of theexample 5-2, and 25° C. in the case of the example 5-3.

[0194] In the results of FIG. 11, it should be noted that the mechanicalproperties shown in each of the examples represent the average of threemeasurements conducted upon three test pieces. Thereby, the effect ofscattering of individual measurement is eliminated.

[0195]FIG. 12 shows the results of the measurement conducted upon thethree test pieces for the example 5-1. Further, FIG. 13 shows themeasurement conducted upon the three test pieces for the example 5-2,FIG. 14 shows the results of the measurement conducted upon the threetest pieces for the example 5-3, FIG. 15 shows the results of themeasurement conducted upon the three test pieces for the example 5-4,and FIG. 16 shows the results of the measurement conducted upon thethree test pieces for the example 5-5.

[0196]FIG. 17 shows the relationship between the load and elongationobserved for the test piece of the example 5-1, wherein the relationshipof FIG. 17 is for one of the three test pieces that has shown the resultclosest to the average. Similarly, FIG. 18 shows the relationshipbetween the load and elongation observed for the test piece of theexample 5-2, wherein the relationship of FIG. 18 is for one of the threetest pieces that has shown the result closest to the average. FIG. 19shows the relationship between the load and elongation observed for thetest piece of the example 5-3, wherein the relationship of FIG. 19 isfor one of the three test pieces that has shown the result closest tothe average. FIG. 20 shows the relationship between the load andelongation observed for the test piece of the example 5-4, wherein therelationship of FIG. 20 is for one of the three test pieces that hasshown the result closest to the average. Further, FIG. 21 shows therelationship between the load and elongation observed for the test pieceof the example 5-5, wherein the relationship of FIG. 21 is for one ofthe three test pieces that has shown the result closest to the average.

[0197]FIG. 22 shows a representative state of fracture of the test piecefor the example 5-1. Similarly, FIG. 23 shows a representative state offracture of the test piece for the example 5-2. Further, FIG. 24 shows arepresentative state of fracture of the test piece for the example 5-3,FIG. 25 shows a representative state of fracture of the test piece forthe example 5-4, and FIG. 26 shows a representative state of fracture ofthe test piece for the example 5-5.

[0198] It should be noted that FIG. 11 summarizes the foregoingexperimental results in FIGS. 12-25.

[0199] Hereinafter, the relationship between the cooling condition andthe mechanical property of the solder alloy will be described withreference to the experimental results shown in FIG. 11.

[0200] First, the effect of cooling process upon the mechanical propertywill be examined for the case where the mold temperature is set to 200°C., based upon the examples 5-1, 5-4 and 5-5.

[0201] Referring to FIG. 11, it is clearly indicated that the solderalloy provides the smallest elongation of 20.44% when the gradualcooling process is employed in which the cooling rate is minimum. Withincreasing cooling rate, the elongation increases such that anelongation of 33.67% is obtained as a result of the natural coolingprocess. When a water cooling is employed, an elongation of 89.48% isobtained. The foregoing results indicate that one obtains an increasedelongation with increasing cooling rate.

[0202] Next, the effect of the mold temperature on the elongation willbe explained based upon the examples 5-1-5-3 in which the naturalcooling is used throughout but with various initial mold temperatures.

[0203] Referring to FIG. 11, it will be noted that the highest initialmold temperature of 200° C., which provides the smallest cooling rate,results in the smallest elongation of 33.67%, while the lower initialmold temperature of 100° C. provides an increased elongation of 137.50%.When the mold temperature is set to 25° C., it is possible to achieve anelongation of 218.33%. This result also supports the conclusion that theelongation increases with increasing cooling rate.

[0204] Summarizing the experimental results above, the mechanicalproperties of a solder alloy can change variously depending upon thecooling rate, even when the composition of the solder alloy is fixed.With increasing cooling rate, the elongation of the solder alloyincreases, and the solder alloy shows the evidence of ductile fracture.

[0205] As will be noted in FIG. 11, the fracture surface of the testpieces that provide a large elongation, as in the case of the examples5-2-5-4, do not exhibit a scale-like pattern that is typically observedin the fracture surface of an Sn42-Bi58 alloy cooled slowly. Further,the microscopic observation of the fracture surface indicates that thereis a coarsening of texture in the examples 5-2-5-4. Thus, it is believedthat such a coarsening is responsible for the increase of the elongationof the alloy.

[0206] As already noted, the remarkable increase of the elongationoccurs not only in the solder alloy containing Sn and Bi, but also inthe alloy of other compositions. Thus, it is believed that such anincrease of the elongation results from the coarsening of texture of thealloy, caused by the large cooling rate.

[0207] It should be noted that such a solder alloy composition having alarge elongation is particularly useful in flexible printed circuitboards in which the conductor patterns including the solder patternsexperience deformation.

[0208] Hereinafter, a soldering process as well as a soldering rig thatcarries out such a soldering process will be described.

[0209]FIG. 27 shows the soldering process that uses the lead-free solderalloy of any of the previous embodiments for soldering an electric orelectronic component upon a substrate such as a printed circuit board.Of course, the substrate is not limited to the printed circuit board.

[0210] Referring to FIG. 27, a step 10 is conducted at first in which aflux is applied to the part of the printed circuit board on which thesoldering is to be made. The flux is applied for improving the wettingby the solder alloy, wherein a suitable flux is selected in view of thecomposition of the lead-free solder alloy to be used.

[0211] Next, in the step 12, a preheating is conducted upon the printedcircuit board for eliminating inhomogeneity of soldering caused bylocalized cooling and associated solidifying of the molten solder alloy.

[0212] Further, a step 14 is conducted subsequently, wherein thepre-heated printed circuit board is dipped in a bath of molten solderalloy of any of the foregoing compositions, and the molten alloy coversthe exposed conductor pattern as well as the lead or electrode of theelectric or electronic components. Thereby, the soldering is achieved.The steps 10-14 are substantially the same as the conventional solderingprocess that uses a lead-containing solder alloy.

[0213] Next, in the following step 16, the printed circuit board ispulled up from the solder bath and cooled by suitable external coolingmeans, such that the solder alloy experiences a rapid cooling orquenching. As a result of such a rapid cooling, the solder alloy showsan improved elongation as explained already. As the external coolingmeans, one may employ a jet of cooling medium such as a coolant gas orvolatile organic solvent. Such a jet of cooling medium can be appliedselectively to the part where the soldering has just been made.

[0214]FIG. 28 shows the construction of a soldering rig 20 according toan embodiment of the present invention for conducting the solderingprocess of FIG. 27. It should be noted that the soldering rig 20 isdesigned primarily to carry out a soldering of a sheet-like orplate-like object such as a printed circuit board. However, thesoldering rig is by no means limited to such a soldering of printedcircuit boards but is applicable to~various soldering processes.

[0215] Referring to FIG. 27, it will be noted that the soldering rig 20includes a flux coating unit 21, a pre-heating unit 22, a soldering unit23, a transport conveyer 24 and a cooling unit 25 that characterizes therig 20 of the present invention, each of which will be explained below.

[0216] The transport conveyer 24 carries a printed circuit board 26placed thereon and transports the same in a direction indicated by anarrow. Further, the flux coating unit 21, the pre-heating unit 22, thesoldering unit 23 and the cooling unit 25 are disposed consecutivelyalong the transport conveyer 24 in the transport direction of theconveyer 24.

[0217] Thus, the flux coating unit 21 applies a flux upon the printedcircuit board 26 and the preheating unit 22 preheats the printed circuitboard 26 thus applied with the flux. Further, the soldering unit 23carries out the soldering by means of the lead-free solder alloydescribed previously.

[0218] After the soldering, the printed circuit board 26 is forwarded tothe cooling unit 25 by the transport conveyer 24. Thereby, the coolingunit 25 rapidly cools the high temperature solder alloy applied by-thesoldering unit 23. As a result of such a rapid cooling, it is possibleto increase the elongation of the solidified solder alloy as explainedalready.

[0219] FIGS. 29-31 show the construction of the cooling unit 25.

[0220] Referring to FIG. 29 showing an example 25A of the cooling unit25 that uses liquid nitrogen as a cooling medium, the cooling unit 25Aincludes a tank 27 for containing liquid nitrogen wherein the liquidnitrogen in the tank 27 is supplied to an evaporator 28 that evaporatesthe liquid nitrogen and forms a low temperature nitrogen gas. The lowtemperature nitrogen gas thus formed, in turn, is supplied along a pipe29 to which one or more gas nozzles 30 are connected. Thereby, the lowtemperature nitrogen is injected upon the location of soldering on theprinted circuit board for cooling the high temperature solder alloy.

[0221]FIG. 30 shows another example 25B of the cooling unit 25, whereinthe cooling unit 25B uses a volatile freon gas as a cooling medium.

[0222] Referring to FIG. 30, the cooling unit 25B includes a tank 31 offreon to which a supply pipe 33 of freon is connected. Further, one ormore nozzles 32 are connected to the pipe 33 for injecting the freonupon the printed circuit board 26 on the conveyer 24 at the locationwhere the soldering has just been made. Thereby, the high temperaturesolder alloy experiences a rapid cooling upon the evaporation of freon.

[0223]FIG. 31 shows another example 25C of the cooling unit 25 that isdesigned to cool a cylindrical or tubular object after soldering. As thecooling unit 25C of FIG. 31 employs the construction of FIG. 29, thoseparts corresponding to the parts shown in FIGS. 29 are designated by thesame reference numerals and the description thereof will be omitted.

[0224] Referring to FIG. 31, the cooling unit 25C includes an annularnozzle element 35 in which a plurality of nozzles 36 are provided. Thenozzle element 35 is disposed in a tube 37 adapted for passing acylindrical object 34 that has experienced soldering, for example bymeans of a soldering iron 38 that uses a lead-free solder alloy 39 ofthe Sn—Bi eutectic system. Thereby, the object 34 is cooled upon passagethrough an inner space of the annular nozzle element 35. As the nozzles36 are disposed with a generally uniform interval on the nozzle element35, the seam of the cylindrical object 34 where the soldering has beenmade, experiences a uniform cooling by the low temperature nitrogen gasinjected from the nozzles 36.

[0225] Further, the present invention is by no means limited to theembodiments described heretofore, but various variations andmodifications may be made without departing from the scope of theinvention.

What is claimed is:
 1. A lead-free solder ally composition containingSn, Ag and Bi, with respective concentrations set such that saidlead-free solder alloy has a melting temperature lower than apredetermined heat-resistant temperature of a work to be soldered.
 2. Alead-free solder alloy composition as claimed in claim 1, wherein saidsolder alloy composition contains Sn with an amount of 96.5×(100−X)/100in wt % and Ag with an amount of 3.5×(100−X)/100 in wt %, wherein Xrepresents the amount of Bi represented in wt %.
 3. A lead-free solderalloy composition as claimed in claim 1, wherein said solder alloycomposition contains Ag with an amount not exceeding 4.0 wt %, Bi withan amount equal to or larger than 1.0 wt %, and Sn with an amount notexceeding 95.0 wt %.
 4. A lead-free solder alloy composition as claimedin claim 1, wherein said lead-free solder alloy composition contains Agwith an amount between 1.0 wt % and 4.0 wt %, Bi with an amount between1.0 wt % and 40.0 wt %, and Sn with an amount between 55.0 wt % and 95.0wt %.
 5. A lead-free solder alloy composition as claimed in claim 1,wherein said lead-free solder alloy composition contains Ag with anamount of approximately 3.3 wt %, Bi with an amount of approximately 5.0wt %, and Sn with an amount of approximately 91.7 wt %.
 6. A lead-freesolder alloy composition as claimed in claim 1, wherein said lead-freesolder alloy composition contains Ag with an amount of approximately 3.1wt %, Bi with an amount of approximately 10.0 wt %, and Sn with anamount of approximately 86.9 wt %.
 7. A lead-free solder alloycomposition as claimed in claim 1, wherein said lead-free solder alloycomposition contains Ag with an amount of 3.0 wt %, Bi with an amount of15.0 wt %, and Sn with an amount of 82.0 wt %.
 8. A lead-free solderalloy composition as claimed in claim 1, wherein said lead-free solderalloy composition contains Ag with an amount of 2.8 wt %, Bi with anamount of 20.0 wt %, and Sn with an amount of 77.2 wt %.
 9. A lead-freesolder composition as claimed in claim 1, wherein said leadfree solderalloy composition contains Ag with an amount of 2.4 wt %, Bi with anamount of 30.0 wt %, and Sn with an amount of 67.6 wt %.
 10. A lead-freesolder alloy composition as claimed in claim 1, wherein said lead-freesolder alloy composition contains Ag with an amount of 2.1 wt %, Bi withan amount of 40.0 wt %, and Sn with an amount of 57.9 wt %.
 11. Alead-free solder powder comprising: a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Sn, Ag andBi, with respective concentrations set such that said lead-free solderalloy has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered.
 12. A lead-freesolder paste, comprising: a lead-free solder powder comprising aplurality of lead-free solder particles each having a generallyspherical shape with a diameter of 20-60 μm; each of said lead-freesolder particles containing Bi with a concentration not exceeding 60.0wt %; In with a concentration not exceeding 50.0 wt %; one or moreelements selected from a group consisting of Ag, Zn, Ge, Ga, Sb and P,with a concentration equal to or larger than 1.0 wt % but lower than 5.0wt %; and Sn as a remaining component of said solder alloy; saidlead-free solder powder being contained with a proportion of 80.0-95.0wt %; and a mixture of an amine halide, a polyhydric alcohol; and apolymer, with a proportion of 20.0-5.0 wt %.
 13. A lead-free solderpaste, comprising: a lead-free solder powder comprising a plurality oflead-free solder particles each having a generally spherical shape witha diameter of 20-60 μm; each of said lead-free solder particlescontaining Sn, Ag and Bi, with respective concentrations set such thatsaid lead-free solder particle has a melting temperature lower than apredetermined heat-resistant temperature of a work to be soldered; and amixture of an amine halide, a polyhydric alcohol and a polymer, with aproportion of 20.0-5.0 wt %.
 14. A lead-free solder paste, comprising: alead-free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Sn, Bi andIn, with respective concentrations set such that said lead-free solderpowder has a melting temperature lower than a predeterminedheat-resistant temperature of a work to be soldered, said lead-freesolder powder being contained with a proportion of 80.0-95.0 wt %; and amixture of an organic acid, a polyhydric alcohol and a polymer, with aproportion of 20.0-5.0 wt %.
 15. A lead-free solder paste, comprising: alead-free solder powder comprising a plurality of lead-free solderparticles each having a generally spherical shape with a diameter of20-60 μm; each of said lead-free solder particles containing Bi with aconcentration not exceeding 60.0 wt %; In with a concentration notexceeding 50.0 wt %; one or more elements selected from a groupconsisting of Ag, Zn, Ge, Ga, Sb and P, with a concentration equal to orlarger than 1.0 wt % but lower than 5.0 wt %; and Sn as a remainingcomponent of said solder alloy; said lead-free solder powder beingcontained with a proportion of 80.0-95.0 wt %; and a mixture of anorganic acid, a polyhydric alcohol and a polymer, with a proportion of20.0-5.0 wt %.
 16. A lead-free solder paste, comprising: a lead-freesolder powder comprising a plurality of lead-free solder particles eachhaving a generally spherical shape with a diameter of 20-60 μm; each ofsaid lead-free solder particles containing Sn, Ag and Bi, withrespective concentrations set such that said lead-free solder alloy hasa melting temperature lower than a predetermined heat-resistanttemperature of a work to be soldered; and a mixture of an organic acid,a polyhydric alcohol and a polymer, with a proportion of 20.0-5.0 wt %.17. A printed circuit board, comprising: a substrate; a conductorpattern provided on said substrate; and a lead-free solder alloycovering said conductor pattern, said lead-free solder alloy containing:Sn, Ag and Bi, with respective concentrations set such that saidlead-free solder alloy has a melting temperature lower than apredetermined heat-resistant temperature of a component to be solderedupon said substrate.
 18. An electronic component, comprising: anelectronic component body; an electrode projecting from said electroniccomponent body; and a lead-free solder alloy covering said conductorpattern, said lead-free solder alloy containing: Sn, Ag and Bi, withrespective concentrations set such that said lead-free solder alloy hasa melting temperature lower than a predetermined heat-resistanttemperature of a component to be soldered upon said substrate.
 19. Anelectronic apparatus, comprising: a substrate; a conductor patternprovided on said substrate; an electronic component mounted upon saidsubstrate in electrical connection with said conductor pattern, saidelectronic component having an electrode projecting therefrom; and alead-free solder alloy connecting said electrode to said conductorpattern, said lead-free solder alloy containing Sn, Ag and Bi, withrespective concentrations set such that said lead-free solder alloy hasa melting temperature lower than a predetermined heat-resistanttemperature of said electronic component.